This is the most reliable source for fresh trimmings. Many butchers sell beef fat for around $1.99/lb , and some may even give it away for free or at a very low cost if you ask.
For immediate cooking use, pre-rendered tallow is widely available at major retailers and online: Grassfed Beef Raw Fat - White Oak Pastures where can i buy beef fat
Many online farms sell raw grass-fed beef fat in bulk. For example, White Oak Pastures offers 2.5 lb packs, while Beaver Brook Ranch sells 10 lb bags of trim fat. This is the most reliable source for fresh trimmings